Chromatography Forum

Hi,
Could you tell me, why grow up the pressure when I have mobile phase with water: water/ACN=50/50, and when I use ACN 100% the pressure is normal?

Good Morming,

The complete answer I do not know, but this link may be of interest to you--it was posted by Jiri Urban:

http://www.chromatographer.com/mobile-phase-viscosity/

The viscosity of acetonitrile - water mixtures changes as a function of the percentage of each solvent present. The higher the viscosity of the solvent mixture, the greater the pressure required to pump the solvent mixture, and raising the temperature lowers the viscosity.

Hi,
Could you tell me, why grow up the pressure when I have mobile phase with water: water/ACN=50/50, and when I use ACN 100% the pressure is normal?

1. Aqueous and mixed aqueous-organic solvents have more dissolved air in them than do pure organic solvents - are you using a degasser or other way to remove dissolved gases before the pump module?

2. Are you purging all four channels (assuming Quaternary pump here) at least 5 minutes per channel at 5ml/minute even if you're only using 1 or 2 channels, important.

3. This relates to #1 as well: if you're using a 1050 model vacuum degasser, the hold-up PER CHANNEL is like 40 ml, so you need to purge/pump through to make sure your mobile phase reaching the pump is really what you expect.